A growing area in the field of printing is known as flexography or flexographic ink printing. Flexographic ink printing is a branch of rotary typographic printing in which the printing is applied to a print-receiving substrate by use of a flexible relief plate with highly fluid, volatile inks which dry rapidly by evaporation. The process was originally limited to printing paper bags, but many inroads have been made in recent years to extend the process to printing various flexible packaging, textiles, newsprint, and other print-receiving substrates.
A flexographic printing ink generally contains three basic types of ingredients denominated as a solvent, a colorant, and a binder. Minor amounts of other additives also may be employed to provide the desired characteristics (i.e., rheology, viscosity, etc.) for the printing ink composition.
In recent years, increased emphasis has been placed on the use of water as the solvent, rather than an oil-based solvent. Carbon black pigment is commonly used as the colorant in both water-based and oil-based black ink formulations.
The binder component of the printing ink composition generally comprises a resin which functions primarily to increase adhesion of the pigment to the substrate, while also often acting as a dispersing medium and carrier. Acrylic polymer lattices have traditionally been employed as binders in water-based printing ink compositions. However, these acrylic binders are relatively expensive. Thus printing ink formulators have actively sought low-cost alternative binders.
One promising source of alternative low-cost binders under evaluation is lignin. Lignin is a complex, high-molecular weight polymer occurring naturally in close association with cellulose in plants and trees. In the paper-making industry, lignin may be recovered as a by-product of the cellulose product.
Commonly assigned U.S. Pat. No. 4,740,591 (which is hereby incorporated by reference) teaches a method for forming an amine salt of lignin via precipitating lignin from the black liquor residue of a kraft pulping process by acidification and reacting the precipitated lignin with an organic amine. In this method the precipitated lignin was acidified to a pH of no lower than about 5 for the wash-water removal of inorganic salts and other impurities. This method yields a lignin salt with a high level of water-solubility.
A method of using this lignin amine salt as a binder for aqueous printing ink compositions is taught in commonly assigned U.S. Pat. No. 4,891,070 (which is hereby incorporated by reference). However, a major problem has recently been discovered in that inks prepared with this type of binder have proven not to be storage stable (particularly at elevated temperatures). Over time viscosity increases and sediments are formed or gel-like inks are obtained which are extremely difficult to dilute to desired press-ready viscosities (commonly about 10 seconds as measured via a #3 Shell Cup or about 20 seconds via a #2 Zahn Cup).
Therefore, it is the object of this invention to provide an improved low-cost lignin amine salt binder for use in producing storage stable, low viscosity flexographic water-borne black ink formulations.